1. Field of Invention
This invention pertains to methods of producing rigid foams and the foams made thereby, particularly polyurethane modified polyisocyanurate foams used for structural laminated board insulation.
2. Prior Art and Other Considerations
Cellular organic plastic foams made with urethane linkages, or made with a combination of both isocyanurate linkages and urethane linkages, are well known in the art. These foams have been made from the catalyzed reaction between polymeric polymethylene polyphenylisocyanate (a.k.a. Polymeric Methylene Di-Isocyanate, or "PMDI") and polyols of various physical and chemical properties. The PMDI has been used either alone, or in a blend with a blowing agent and (optionally) with a capped silicone surfactant. Such a blend utilizing PMDI has traditionally been called the "A-Blend".
In order to form good cell size, good cell distribution, and good cell-wall construction, it has sometimes been preferred to add other "plastic foam cell modifiers" to the foam formulations. Often, it has been preferred to add these other agents to the polyol mixture. These foam cell modifiers include, but are not limited to: propylene carbonate, dispersing agents, organic surfactants, predominantly silicone surfactants, nucleating agents, fire retardants, and expansion agents. This blend including the polyol(s), expansion agent(s), and catalyst(s), has traditionally been called the "B-Blend".
As used herein, the term "expansion agents" includes blowing agents and frothing agents. Moreover, as used herein, a blowing agent is a substance which is either produced, or becomes a gas, subsequent to the first of several chemical reactions. Many blowing agents have boiling points in the range from about 10.degree. C. to about 50.degree. C. On the other hand, CO.sub.2 is considered a blowing agent since, although it has a boiling point outside this range, it is produced by an isocyanate reaction. A frothing agent is a substance which is a liquid under sufficient pressure, then when released from pressure containment, accordingly produces gas-filled cells in foam prior to the initial chemical reaction.
It has been considered important to keep the viscosity of each mixed blend about equal to the other blend. The rule of thumb has been to keep both the A-Blend and the B-Blend in the range of 250 cps to 1500 cps, and to run the chemical blends at about 60.degree. F. to about 70.degree. F. just prior to mixing. (All viscosities herein are "centipoise" taken on a Brookfield viscometer.)
Prior art insulation thermosetting foams have been primarily "blown" or expanded by the use of CFC-11 (trichloromonofluoromethane). Some minor use of CFC-12 has also been used, as explained below. Due to environmental considerations, both CFC-11 and CFC-12 have fallen into disfavor. Most commercial foam producers have historically formulated all of their foam formulae around CFC-11 properties. These properties of CFC-11 affecting the foam formulae include the boiling point, the solubility parameters, the number of molecules per unit weight, the latent heat at boiling point, and the rate of membrane permeability, of CFC-11. The commercial foam laminate producer has learned to operate the continuous laminating foam board process based upon these known physical properties.
There are two general phenomena which must take place in concert with each other in order to make quality structural foam laminates at a reasonable price. These two phenomena are (1) the expansion of the foam and (2) the foam chemical (polymerization and cross-linking) reactions. The expansion of the foam is measured volumetrically; the chemical reactions are measured in terms of solidification. As used herein, the "degree of completion of expansion" means the degree of completion (with reference to the ultimate potential expansion) of the foam expansion at any point in time. The "degree of completion of the chemical reactions", or comparable phraseology, means the degree of completion (with reference to the ultimate potential degree of polymerization and cross-linking) of the foam chemical reactions at any given point in time.
It is critical that the chemical reactions have enough energy input to proceed to a substantially completed stage. This is especially crucial to foam board laminators who must depend upon a high degree of the trimerization reaction of three PMDI molecules to form the isocyanurate linkage. The most popular plastic foam insulation used for building construction is polyurethane modified polyisocyanurate foam. The only economical way this foam insulation can meet the stringent fire resistance requirements of building codes is to form high levels of isocyanurate group cross-linking.
Likewise, it is important for the successful production of foam board lamination to have the degree of completion of the many complex chemical reactions of the thermosetting polymerization timed with the degree of completion of foam expansion. Additionally, these reactions must be finished quickly enough to obtain the full thickness and to maintain the planned thickness of a laminated board once it leaves a continuous double-belt laminator; at the desired density.
When CFC-11 and CFC-12 are ultimately replaced by alternate blowing and/or frothing agents, the prior art techniques of creating energy input to effect the critical timing of expansion versus reactions will not suffice. Some new frothing and blowing agents have detrimental properties that interfere with exothermic heat energy. Moreover, these new agents demand more energy to function as expansion agents. Alternative blowing agents of this type include hydrochlorofluorocarbons, or partially hydrogenated chlorofluorocarbons, (referenced by the contraction "HCFCs"); as well as the non-chlorine containing fluorocarbons, called hydrofluorocarbons, or just "HFCs". All the physical properties mentioned above (including the boiling point, the solubility parameters, the number of molecules per unit weight, the latent heat at boiling point, and the rate of membrane permeability) differ for HCFCs and HFCs as opposed to CFC-11 and CFC-12. As will be shown, these physical properties are detrimental to both heat energy utilization and the timing of the reactions with the degree of completion of expansion.
For example, both HCFC-123 and HCFC-141b have higher boiling points than CFC-11. The CFC-11 boils at 74.9.degree. F. (23.8.degree. C.); while HCFC-123 boils at 82.2.degree. F. (27.9.degree. C.), and HCFC-141b boils at 89.6.degree. F. (32.0.degree. C.). The higher boiling point means the start of the expansion of foam requires more heat energy input than prior art methods. Using prior art methods, these two new HCFCs naturally slow down foam expansion. Slow expansion of the foam allows the chemical reactions to create solidification prior to cell expansion, which causes a high foam density, i.e., low insulating properties.
Another detrimental effect of some new expansion agents is the cooling effect caused by the partial evaporation of low boiling point products. For example, monochlorodifluoromethane, CHClF.sub.2, or HCFC-22, boils at -41.4.degree. F.(-40.8.degree. C.), meaning some of the product added will evaporate as soon as it is released to atmospheric pressure, and thus will cool the polymer mixture. Another new potential blowing agent in this category, CH.sub.3 CClF.sub.2, or HCFC-142b (monochlorodifluoroethane), has a boiling point of +14.4.degree. F., or -9.8.degree. C.
It has been discovered that the cooling effect of an evaporating frothing agent reduces the exothermic heat generated by the urethane chemical reaction. To a large degree, the exothermic heat from the urethane reaction is the main heat energy source for the trimerization reaction. It is well known that high levels of heat energy are needed to complete the trimerization reaction which causes the PMDI to form into the isocyanurate linkage. A lack of trimerization causes product failures from the loss of dimensional stability and from excess flammability.
U.S. Pat. No. 4,572,865 teaches the production of polyisocyanurate foams using CFC-12, dichlorodifluoromethane, CCl.sub.2 F.sub.2, which boils at -21.6.degree. F. (-29.8.degree. C.), as a frothing agent. While U.S. Pat. No. 4,572,865 does not specifically mention the cooling effect of using CFC-12, it is well known that this frothing agent does create evaporative cooling in rigid foam production. Other than possibly using high oven temperatures, U.S. Pat. No. 4,572,865 fails to teach any chemical reaction to make up the loss of exothermic heat energy which is taken away by the evaporative cooling of the frothing agent, CFC-12.
As mentioned above, CFC-12 has fallen into disfavor. The only practical HCFC to replace CFC-12 as a frothing agent is HCFC-22, CHClF.sub.2, monochlorodifluoromethane. This compound boils at -41.4.degree. F.(-40.8.degree. C.), meaning it boils more easily and cools much faster than does CFC-12. Without a way to compensate for the loss of exothermic heat due to the cooling effect of evaporating HCFC-22, the trimerization reaction would be extremely difficult to effect, if not impossible.
The strong solvent action characteristic of some of the new blowing agents is detrimental if used with methods of the prior art. To a large degree, these new agents are much stronger solvents in both B-Blends and A-Blends than were the CFC blowing agents of the prior art. The increased solubility causes dramatic decreases in blend viscosities. When the viscosity of the foamable blends gets too low, the resulting mixture of A-Blend (primarily PMDI) with B-Blend (primarily polyol) will form cells with thin walls and thick intercellular struts. This creates a foam which is poor insulation. Very small cell diameters (microcellular), with the cells having closed walls and thin struts, all at the proper density, are desired for good insulation properties. To create good cellular walls in the cellular foam matrix, the viscosity of the final foaming mixture must be high enough to restrain "drainage" from the cell wall into the cellular strut. Another need for higher viscosity polyols arises from the use of frothing agents. When a rapid frothing action occurs in a low viscosity liquid, the cell walls rupture creating an open celled foam. This is a common practice in producing flexible foam, as explained below.
Thus it is seen that prior art methods of continuous lamination processing must be significantly changed to utilize HCFCs in order to be commercially successful. New methods are needed to compensate for the detrimental effect upon exothermic heat generation, and to maintain the timing that is needed in commercial foam production to balance the speed of expansion with the speed of chemical reactions.
It is therefore an object of the present invention to provide an improved method for the production of a rigid thermosetting plastic foam insulation, which method provides an increased amount of exothermic heat.
An advantage of the present invention is the provision of a method which overcomes the negative effects of evaporative cooling from low boiling point frothing agents.
An advantage of the present invention is the provision of a method which not only overcomes the negative effects of evaporative cooling from low boiling point frothing agents, but also conveniently maintains the rate of expansion when utilizing higher boiling point blowing agents.
It is another advantage of the present invention to provide a method whereby a frothing agent having a lower boiling point than used in prior art foams, as well as blowing agents with higher boiling points, can be used together and still maintain the temperatures needed for the completion of the trimerization reaction as well as maintaining the timing of the speed of foam expansion with the speed of chemical reactions.
It is a further advantage of the present invention to provide an improved cell structure in rigid plastic foam insulation by utilizing smaller organic molecules in solution than previously used as a nucleating agent in the process.
Yet a further advantage of the present invention is the provision of a method that compensates for the rapid expansion of a frothing agent by maintaining strong cell walls and a high percentage of closed foam cells.
Yet another advantage of the present invention is the provision of a method that compensates for the strong solvent action of some new blowing agents and still maintains good cell wall formation.
It is still another advantage of the present invention to provide an improved structural laminated foam board insulation at a cost lower than would be possible by using HCFC blowing agents alone, by utilizing at least some CO.sub.2 blowing agent.
A further advantage of the present invention is the provision of an improved structural laminated foam board insulation at a cost lower than would be possible by using blowing agents plus utilizing at least some CO.sub.2 blowing agent by additionally utilizing a frothing agent.
A further advantage of the present invention is the provision of a strong, economical, closed cell foam insulation which is characterized by a high degree of fire resistance and a high resistance to thermal conductivity.